The Seversky Aircraft Company, which in 1939 changed its name to Republic, constructed a range of private venture, one-off variants of its P-35 design, featuring different powerplants and enhancements, designated AP-2, AP-7, AP-4 (which flew after the AP-7), AP-9, XP-41. The series included a carrier-based version designated the NF-1 (Naval Fighter 1) that was also built. The most significant of these was the AP-4, which served as the basis for future Seversky/Republic aircraft. It featured fully retractable landing gear, flush riveting, and, most significantly, a Pratt & Whitney R-1830-SC2G engine with a belly-mounted turbo-supercharger, producing 1,200 hp (890 kW) and good high-altitude performance. The turbo-supercharger had been refined by Boeing as part of the development program for the B-17 Flying Fortress, and the improved performance it offered was of great interest to other aircraft manufacturers.The XP-41 and sole AP-4 were nearly identical, although the AP-4 was initially fitted with a large prop spinner and a tight-fitting engine cowling, as a testbed to evaluate means of improving the aerodynamics of radial-engined fighters, following similar experiments with the first production P-35. The AP-4's big spinner was later removed and a new tight cowling fitted. Unsurprisingly, these measures led to overheating problems. On 22 March 1939, the engine caught fire in flight, the pilot had to bail out, and the AP-4 was lost. Despite the loss of the prototype, the USAAC liked the turbo-supercharged AP-4 demonstrator enough to order 13 more in May 1939, designating them YP-43.

The YP-43 differed from AP-4 in having a "razorback" fuselage with a tall spine extending back from the canopy. The engine air intake was moved from the port wing to under the engine resulting in the distinctive ovoid cowling. The aircraft was powered by an R-1830-35 14-cylinder air-cooled radial engine with a General Electric B-2 turbo-supercharger generating 1,200 hp and driving a three-blade variable-pitch propeller. Armament consisted of two synchronized .50 in (12.7 mm) machine guns in the cowl and a single .30 in (7.62 mm) machine gun in each wing.The first of 13 YP-43s was delivered in September 1940, the last in April 1941. Early testing revealed a strong tendency to yaw during takeoff and landing rolls, fixed by redesigning the tailwheel. Although the aircraft exceeded the initial USAAC performance requirements, by 1941 it was clearly obsolete, lacking maneuverability, armor, or self-sealing fuel tanks. The USAAC felt the basic P-35/P-43 design had exhausted its reserves for further improvement in performance and shifted its interest to the promising P-47.

Production aircraft identical to the YP-43 prototypes, were designated "Lancer" and were delivered between 16 May and 28 August 1941. Ongoing delays in the P-47 program resulted in USAAC ordering an additional 80 P-43J, with Pratt & Whitney R-2180-1 Twin Hornet engine rated at 1,400 hp (1,000 kW). The engine promised better high-altitude performance, and armament was upgraded with 0.50 in machine guns replacing the 0.30 in in the wings. The USAAC was sufficiently interested to assign the AP-4J variant an official designation P-44 Rocket. Combat reports from Europe indicated that the new type was already obsolete, consequently, the entire order was canceled on 13 September 1940, with no prototypes built.Alexander Kartveli and his team focused their efforts on the advanced AP-10/XP-47 which eventually became the fabled P-47 Thunderbolt. When the Pratt & Whitney R-2800 engine intended for the new P-47 was not yet available, it was decided to order 54 P-43s to keep the Republic production lines operating. An additional 125 P-43A-1s were ordered for China through the Lend-Lease program, originally intended to equip the Third American Volunteer Group (AVG). These initially differed in the Air Materiel Command specification from earlier P-43s in being armed with two 0.50 in machine guns in each wing and no fuselage guns, and having rudimentary armor and fuel tank protection.This would have required a series of serious engineering changes. Reality intervened: actually, as delivered, the P-43A-1 had the same armament layout as the P-43As: four .50 in machine guns, two in the cowl and two in the wings. Externally, they were identical, and only the serial numbers distinguishes a P-43A from a P-43A-1. Many of these aircraft were fitted with cockpit armor before shipment westward from California in crates; evidence is murky whether the cockpit armor came from Republic or was cobbled together after delivery.By 1942, a total of 272 P-43 and its variants were built, a remarkable number considering the original intention was to not build any.